The present invention is directed to electrical amplifier apparatuses, and especially to electrical amplifier apparatuses embodied in programmable gain amplifiers. Programmable gain is a useful feature for an amplifier apparatus for many reasons. An important reason is that a manufacturer may produce a particular model of a product that may be employed by customers for a variety of applications. By producing such multi-use products, a manufacturer may be able to carry fewer individual parts in inventory, and a customer may be able to finely tune a product to provide peak performance in a particular device.
By way of example and not by way of limitation, electrical amplifier apparatuses embodied in programmable gain amplifiers are used in video output buffer circuitry in video decoder applications. Many electronic applications are presently experiencing increased use of digital technology. In order to provide back compatibility from newer digital models to older analog models, such as in video applications, a video decoder may be provided for digitizing base-band analog video formats into digital component video signals. Such back compatibility is useful, by way of example and not by way of limitation, in television (TV), video cassette recorders (VCR), digital video disc recorders (DVDR) and camcorders. To properly monitor decoding process, the electrical amplifier apparatuses can be used as a video output buffer circuit to provide maximum amplitude that can be programmed for different applications.
Others have addressed this design requirement previously. In J. J. F. Rijns, “CMOS Low-Distortion High-Frequency Variable-Gain Amplifier”, IEEE Journal of Solid State Circuits, Vol. 31, pp. 1029–1034, July 1996 (hereinafter referred to as “Rijns”), Rijns discloses an analog input signal being first transformed into a current signal, and the current flows through a programmable resistor to achieve a variable gain. Such a transconductance approach is sufficient for internal driving of devices on-chip. However, such an approach may not suffice when driving a large off-chip capacitance, e.g., on the order of 10 pF. In L. Moldovan and H. H. Li, “A Rail-to-Rail Constant Gain Buffered Op-Amp or Real Time Video Applications”, IEEE Journal of Solid-State Circuits, Vol. 32, pp. 169–176, February 1997 (hereinafter referred to as “Moldovan”), Moldovan discloses a rail-to-rail buffer amplifier that cannot have a variable gain.
There is a need for a variable gain amplifier apparatus capable of driving large off-chip capacitive loading with little distortion.
There is a need for a method for buffering a signal with a programmable gain that is appropriate for driving large off-chip capacitive loading with little distortion.